Sep. 18, 2013

In 2011, while doing reconnaissance work on the Lateral West Fire at the Great Dismal Swamp in Virginia, firefighter Greg Sanders encountered a menacing scene: a wall of flames spewing heat and smoke, and booming with exploding logs. “It sounded like cannon fire coming out of the area,” he says. From his vantage point several hundred yards away, he also spied flaming twists known as fire whirls rolling out from the edge of the blaze.

Picture a burning dust devil—that’s essentially what a fire whirl is, says Michael Durfee, a fire management officer with the U.S. Fish and Wildlife Service. Both dust devils and fire whirls form at the ground, starting with air that’s been heated by the sun or by fire. That heated air rises, forming a convective column. A little wind is often enough to make the shaft of rising air spin, like a water eddy in a stream. The result is a vortex that sucks surrounding flames and ash skyward, creating a cobra-like swirl similar to the one pictured above.

These vortexes also suck surrounding air in, increasing wind speed and intensity. “They sound like a tornado; you’ll hear [them] howl,” says Durfee, who worked as a fire behavior analyst on the Lateral West Fire. While wind speeds in the most destructive tornadoes can reach more than 200 miles per hour, the biggest fire whirls top out around half that, he says.

Still, that’s enough to do some serious damage. According to a U.S Forest Service manual on fire behavior, the Great Chicago Fire of 1871 ignited fire whirls that lifted burning planks more than a quarter mile ahead of the main blaze, “which contributed greatly to the spread and destruction of the fire.” On the very same day, a fire 250 miles to the north, in Peshtigo, Wisconsin, created a whirl that was “strong enough to lift a house off its foundations.”

Because they behave unpredictably, fire whirls present a big problem in fighting wildfires. “Fire whirls would be up there in the big headache category,” says Durfee. “They can creep up and explode on you rapidly within minutes,” spreading fire by ejecting embers and even burning tree limbs, and by separating from the main blaze, he says.

Fire whirls can dissipate just as quickly as they form, especially if they stray too far from the fire and lose their energy source. But on that day two years ago in Virginia, a big one spun long enough for Sanders to whip out the camera he keeps in his pack and snap a photo that left a burning impression.

About Andrew P. Han

Andrew is a New York-based freelance writer with a graduate degree from NYU's Science, Health, and Environmental Reporting Program. His favorite graph is the Hertzsprung-Russell diagram. Follow him @HanAndrewP

The views expressed are those of the author and are not necessarily those of Science Friday.

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